Add test for scalar multiplication of Vector3

Also fix the operator* definition with appropriate use of std::is_arithmetic to
allow scalar multiplication in either order (pre or post) without triggering
ambiguous overload errors.

libs/math/Vector3.h

@@ -423,16 +423,22 @@ class BasicVector3
 };
 
 /// Multiply vector components with a scalar and return the result
-template <typename T, typename S>
+template <
+    typename T, typename S,
+    typename = typename std::enable_if<std::is_arithmetic<S>::value, S>::type
+>
 BasicVector3<T> operator*(const BasicVector3<T>& v, S s)
 {
     T factor = static_cast<T>(s);
     return BasicVector3<T>(v.x() * factor, v.y() * factor, v.z() * factor);
 }
 
 /// Multiply vector components with a scalar and return the result
-template <typename T>
-BasicVector3<T> operator*(T s, const BasicVector3<T>& v)
+template <
+    typename T, typename S,
+    typename = typename std::enable_if<std::is_arithmetic<S>::value, S>::type
+>
+BasicVector3<T> operator*(S s, const BasicVector3<T>& v)
 {
     return v * s;
 }

test/math/Vector3.cpp

@@ -117,6 +117,17 @@ TEST(MathTest, SubtractVector4)
     EXPECT_EQ(v1 - v2, Vector4(-0.125, 92, 449.5, -93.5));
 }
 
+TEST(MathTest, ScalarMultiplyVector3)
+{
+    Vector3 vec(2, 4, -5);
+
+    EXPECT_EQ(vec * 2, Vector3(4, 8, -10));
+    EXPECT_EQ(2 * vec, Vector3(4, 8, -10));
+
+    EXPECT_EQ(vec * 0.5, Vector3(1, 2, -2.5));
+    EXPECT_EQ(0.5 * vec, Vector3(1, 2, -2.5));
+}
+
 TEST(MathTest, Vector3AsCArray)
 {
     Vector3 vec(256, -10, 10000);